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A couple of questions about antennas, bandwidths and power

  1. Jan 10, 2019 #1
    1. Does it take more electrical energy to transmit HF and lower frequencies than it takes to transmit VHF and above?
    2. Are there any wavelengths beyond radio waves and gamma rays?
    3. Is it true that the lower the frequency, the lower the amount of data that's being sent? I read on this wiki that military submarines can only send around 300 bit/s – or about 35 eight-bit ASCII characters per second when sending in ELF, and a few characters per minute when sent in ELF. Is there a chart or an article somewhere showing if there's any relation?
    4. If I have an antenna that's ~1cm, will it still be able to pick up and send HF and lower signals?
    5. Isotropic antennas can't really be achieved, but what types of antennas are most like isotropic anyway?
    Thanks a lot for reading. It's ok if you can only answer a few, I just need some help with this.
     
  2. jcsd
  3. Jan 10, 2019 #2

    phinds

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    The electromagnetic spectrum is theoretically infinite, so of course there are.
    Well, sort of. It depends on the encoding technique but in general yeah.
     
  4. Jan 10, 2019 #3

    Borek

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    Technically everything beyond X-rays is called gamma, so while there is no limit, nothing can be "beyond gamma".
     
  5. Jan 10, 2019 #4
    Not as far as I know. Huge transmitters are typically used for the submarine communications at ELF and VLF but I think that's because they're trying to generate a signal that will travel around the world.

    No. Radio waves are the name for everything at the low frequency end of the spectrum, and gamma rays are the name for everything at the high end.

    It's mostly because of bandwidth. A signal doesn't occupy just one frequency, it occupies a range of frequencies. Roughly speaking, the bandwidth is on the same order as the data rate. For instance, frequencies in the FM band (with numbers like 90.9 or 105.3) are spaced at intervals of 0.2 MHz or 200 kHz. They can accommodate a data rate of about that size without interfering with each other.
    But if your base frequency is only 10 kHz, you can't fit a 200 kHz bandwidth around that. Your bandwidth is going to be more like 1 kHz or so, and so that's your data rate.

    Yes, just not very efficiently. The signal levels will be lower.

    Whip antennas (single vertical piece of metal) give an omnidirectional pattern which is uniform in all horizontal directions and has a pretty good spread vertically.
    https://en.wikipedia.org/wiki/Omnidirectional_antenna
    I think I've seen omnidirectional antenna specs with gains of about 2 dB, which translates to about a 60 degree vertical beamwidth. Gain and directionality are related. Gain is power relative to isotropic power. So a gain of 10 dB, a factor of 10 above isotropic, is achieved by limiting the energy to 1/10 of the sphere. A perfectly isotropic antenna would have a gain of 0 dB.

    I'm no antenna expert. If you search on "low gain antenna" you'll find antennas designed to be nearly isotropic. I just did that and saw a picture of a drone, which reminds me that drones are one application where you need these. It needs to be able to talk to the home base in any direction.
     
  6. Jan 10, 2019 #5

    Tom.G

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    1) Usually the other way around, more energy for the higher frequencies because the vacuum tubes or transistors are less efficient at the higher frequencies.

    There is another practical aspect to this. The signal loss thru a medium is proportional to the thickness of the medium measured in wavelengths. That is why satellite TV dishes are always mounted outdoors with a direct line-of-site to the satellite. The microwave frequencies used don't make it thru building material very well. This is also one of the reasons submarine communication is done at extremely low frequencies. The radio waves have to get thru a fair amount of sea water, which has a fairly high loss for radio waves.

    4) Antennas work best when they are near a multiple of the wavelength in use (don't ask why, it gets complicated), or alternatively ½ or ¼ of the wavelength if you don't need super-high efficiency. (these shorter ones are fine for many uses, like the 'rabbit ears' TV antennas). Any length of wire will pick up or transmit any radio frequency, just not much at all if there is a large difference between the wire length and the wavelength.

    Hope this helps some.

    Cheers,
    Tom
     
  7. Jan 11, 2019 #6

    sophiecentaur

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    1. This question involves many different factors. It's basically about Efficiency and the efficiency of each component of the chain. Power Amplifiers used in RF transmitters are in the region of, say 50% to 90% but that depends on the actual type of signals being transmitted. You then need to consider the efficiency of the transmitting antenna system. VHF and UHF antennae can be made more directive than MF and HF so they can make more use of the available transmitter power by pointing it where it's needed. That's effectively an efficiency matter. VLF antennae need to be very large and, even then they tend to be inefficient; hundreds of kW of transmitter power may mostly end up warming up the ground or the sea.
    So it's one of those "How long is a piece of string" questions, I'm afraid and you need to specify a bit more detail - there isn't a simple "High frequency good Low frequency bad": sort of answer.
    3. This is a good question - low frequency systems cannot carry much information because the information bandwidth cannot be significantly higher then the 'operating' frequency and that channel space is all used up and can't be used again within hundreds of miles. RF carriers with much high frequency allow more information to be transmitted by using 'channels' of a given bandwidth and different carrier frequencies. Submarine communication is very data limited but it has to involve very low carrier frequencies because that's the only band that will penetrate under the sea. Optical comms systems can be used to carry many channels of many GHz each down the same fibre because the 'carrier frequencies' are Hundreds of THz.
    4. A 1cm antenna can be made to receive MF signals quite effectively by using a 'Magnetic' antenna, consisting of a child wound round a Ferrite core. 1cm is a bit on the small side, true, but tiny MF receivers can be bought with ferrite antennae which are only two or three cms long. Unfortunately, the low efficiency of such an antenna would make it unsuitable for transmitting MF - it would melt before a useful power level of RF could be radiated.
     
  8. Jan 11, 2019 #7

    tech99

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    Let's make a fair test to find out what happens if we change wavelength. Take a transmitting dish and a receiving dish, each 1 sq metre, and place them 10km apart with a clear view between them.
    Let's use a wavelength of 0.01m. The famous formula of Friis tells us the ratio of transmitter power to receive power.
    [​IMG]
    Pr = Pt Ar At / d^2 lambda^2
    Pr = 1*1*1 / 10^8 * 0.01^2
    Pr = 10^-4 watt. (100 microwatts)
    Now change wavelength to 0.001m.
    Pr = 10^-2 watt (10mW)
    So if everything is kept the same we need more power for longer wavelengths - lower frequencies.
    But of course, many other factors change between typical LF and SHF systems, including the effect of the ground, atmospheric noise, data rate, type of path etc.
    We have coverage of all England with a 500kW LF transmitter, but a 100 kW VHF transmitter only gives 30 miles radius.
    There is a particular problem with typical situations in the 30 to 1000 MHz region, where there is a strong ground reflection which tends to cancel the signal. This makes the signal fall off much faster with distance
     
  9. Jan 12, 2019 #8

    berkeman

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    Just to add to the reply by @sophiecentaur -- We discussed this for the case of AM radio antennas back in November in your earlier thread:

    https://www.physicsforums.com/threads/does-the-size-of-an-antenna-matter.960025/#post-6088212
     
  10. Jan 12, 2019 #9

    sophiecentaur

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    Those frequencies are limited more or less to line of sight and the Earth's curvature limits the possible range - except under rare propagation conditions. Putting a transmitting antenna on a high mast gives improved gain ('height gain'). The vertical radiation pattern VRP) is often tailored (broadcast, high gain antennae) to point just below the horizon. When you can get it right, the received signal level is far from an inverse square law pattern as, at low angles the VRP drops to a low value and more power is directed to the edge of the service area.

    Ground wave propagation (that was a big surprise in the early days) works well for MF and LF for Vertical polarisation but skywave propagation causes skywave fading and skywave interference once the Sun goes down. BBC Radio 4 LF is hardly useable in the West Country at night and the two supplementary LF stations in Southern and Northern Scotland introduce further problems the the 'Mush Areas'. Those low frequencies certainly have their uses but the spectrum space is limited and I suspect they may fall into disuse except for emergency use and many people these days never use the AM channels - wouldn't even know that their receivers can pick it up.
     
  11. Jan 12, 2019 #10
    First off, thank you to everyone who replied! It's a lot clearer to me now and I appreciate reading your answers.

    Does that mean there is more past 0hz?

    I'm sorry, I wanted to be a little more specific for something that I'm looking into. Thank you for helping though!
     
  12. Jan 13, 2019 #11

    Borek

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    You mean negative frequencies? No.

    Still, with frequencies like 0.1 Hz, 0.01 Hz, 10-10 Hz and so on there is no limit to how low the frequency can be. They will all count as radio waves.
     
  13. Jan 13, 2019 #12

    sophiecentaur

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    Although they are frequently used, in the mathematical sense, in signalling theory.
    I am not sure that one would refer to the EM radiation from mains electricity supplies as "Radio Waves". (Someone may well quote me an example here)
    But what to call them is actually pretty irrelevant. The general term EM Waves would cover the lot. How we refer to different parts of the EM spectrum is largely only of concern to people who need to answer elementary Science Exam questions. If it's not in the text book then don't worry about it.
     
  14. Jan 13, 2019 #13

    davenn

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    why not ... it's an EM radiation generated by the acceleration of electrons ( charges) ....
    It radiates and can be picked up on a radio

    seems to qualify as a radio signal to me :smile:
     
  15. Jan 14, 2019 #14

    sophiecentaur

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    Yes - that could be one way to define radio waves but when is a radio not a radio? Anything capable of detecting EM waves could be called a radio if that's what we want. I always think in terms of the way that the different waves are detected. I.R up to gamma are (or at least have been) detected non-coherently by the direct effect of photons. Otoh, Radio waves can be detected coherently with electronics and their individual photons have low energy, below the noise level.
    That classification works fairly well except that, in practical terms, there will always be a boundary line somewhere above current RF frequencies where it may be possible to make a 'radio set' to demodulate a coherent signal. Lasers may actually be doing this already but I don't know enough about that they're actually doing to know how they differ or not from a superhet radio receiver.
    But I do see an inherent lower frequency for defining radio waves and that is to do with the nature of the fields that we can detect. 50Hz EM interference is essentially Near Field. The wavelength is as great as the dimensions of a country and what can be measured is not a space (radio) wave and the impedance is not 377Ω on Earth.
    I wish I hadn't got involved in this classification argument because it's always down to personal taste in the end.
     
  16. Jan 14, 2019 #15
    In the valve (tube) radio world, the problem of noise from nearby mains cables is sometimes referred to as ‘RF interference’.

    Having said that, you could then define ANY frequency as radio frequency.
     
  17. Jan 14, 2019 #16

    sophiecentaur

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    I agree that the high harmonics and other frequencies would be rightly called RF interference because the electronics picks it up. Whether the 50 / 60Hz would be Radio Waves is another matter. Mains Hum is due to local magnetic and/or electric fields but is it a propagating wave?. Where would Audio Frequency come into the dreaded classification story?
    If you want to any frequency RF then you may just as well use the term EM because it is just as open and non-specific. There is really no definitive answer but it's the sort of thing that not-too-well-informed educationists seem to find very important and they are always putting questions in exam papers and causing alarm and despondency amongst the confused student body. Also, we read of Ultraviolet Light and Infra red Light, too. Isn't Light the stuff we can see??
     
  18. Jan 14, 2019 #17
    It depends... A hawk can see ultraviolet, and a cat can see infrared. Very easy to get anthropocentric - we’re just one of millions of species with eyes.

    I suppose it’s best not to get too bogged down when putting boundaries into continuous spectra.
     
  19. Jan 14, 2019 #18
    I guess if you have a charge that moves from one place to another and back again, the movement of its e-field will propagate away..?

    My little Fluke pen can ‘sniff’ AC in a nearby wire, for example.
     
  20. Jan 14, 2019 #19

    tech99

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    When your Fluke senses an E field near an AC conductor, that is not radiated energy. Most of the energy stored in the E-field will return to the conductor on the next half cycle. Radiated energy is that which leaves the conductor and is gone for ever. Even with a dipole antenna, the stored fields are about ten times greater than the radiated field. The local fields are referred to as the reactive near field. Radiation from power lines is extremely small due to the huge wavelength.
     
  21. Jan 14, 2019 #20

    Tom.G

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    And then there are Whistlers. They are low frequency (≅1kHz and up)electromagnetic waves with lightning as the source. The signals travel between the N. & S. hemispheres guided by the Earths magnetic field lines. They suffer frequency dispersion in their travels, hence the Whistler name. I have heard them using an audio amplifier and a long wire as an antenna; you have to be far from power lines though.

    Once I tried it in a city park with poor results. The antenna was strung out on the ground but it was also picking up footsteps! The city was shredding trees and using the wood pellets for mulch. It turned out there was enough charge generated by walking on the partially decomposed wood chips to drowned out any signal. I've always wondered whether it was a triboelectric or a piezoelectric charge being generated; or maybe both.

    see: https://en.wikipedia.org/wiki/Whistler_(radio)

    Cheers,
    Tom
     
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